Prop Sizing
 

Hello,

I am working with a converted weedeater engine but my question only pertains to the prop. I am spinning a 15x8 up tp 12000 RPMs on the ground, much to high. I would like to switch to a 3 blade to save some ground clearance. I know you drop an inch when you switch to 3 blade to stay the same RPM, but what about lowering the RPMs is there any kind of guideline there? I think I should drop the RPM down atleast to 9500 on the ground if not lower. What prop would you suggest trying? I dont want to buy an entire handful of props at $15 a prop.

Thanks, Erik

No clue what type plane you're working on, so I'll just go on the fact I don't like more than 8" of pitch as a rule (fly mostly acrobatic) and my WAG will use that pitch. To drop your rpm 2K or so, you'll need to increase your prop diameter by a couple inches, to let's say 17". Now, you can convert that 17x8 2 blade that should turn somewhere around 10k, to 3 blade as you wish. Probably a 16x8? Again, all purely hypothetical. Your results probably WILL vary, but you should be a LOT closer than you are now....

What cc is this weedwacker in question? judging RPM it could probably swing an 18x8.

I it is a 25cc, I did fairly extensive mods to the motor. I was thinking about trying a 16x8 3 blade as thats what is in stock at my LHS. The model is a Pitts Ultimate Bipe. Thanks for the suggestions!

Just for reference I run a M/A 16x10 on a black horse stuka with a DLE 30, doesn't seem to rev out too high, not sure I haven't tested it really as it usually flies at 1/4 to 1/3 throttle
Edit, that 16x10 is a 3 blade

Eric if you are wanting to go to a smaller diameter than the 15" that you are currently using (if I read correctly) to say a 14" 3 blade prop then you are going to need a significant increase in pitch to get the rpm's down. The usual theory for moving from 2 blades to 3 blades is to reduce 1" in diameter and increase 1 to 2" in pitch to maintain a similar rpm. You are already significantly undersized on your prop diameter already so you will need a big increase in pitch to get the prop speed down. A large jump in prop pitch will greatly increase your flight speed. I would think a 14x12 to 14x16 may be needed to get the rpms down to the 7k to 8k range. The speed created from that much pitch may make things interesting

CK1, I am not trying to decrease my prop size just keep from getting too much larger than I was using but increasing thrust and reducing rpms if all that is possible.